A known autostereoscopic display device comprises a two-dimensional liquid crystal display panel having a row and column array of display pixels (wherein a “pixel” typically comprises a set of “sub-pixels”, and a “sub-pixel” is the smallest individually addressable, single-color, picture element) acting as an image forming means to produce a display. An array of elongated lenses extending parallel to one another overlies the display pixel array and acts as a view forming means. These are known as “lenticular lenses”. Outputs from the display pixels are projected through these lenticular lenses, which function to modify the directions of the outputs.
The lenticular lenses are provided as a sheet of lens elements, each of which comprises an elongate partial-cylindrical (e.g. semi-cylindrical) lens element. The lenticular lenses extend in the column direction of the display panel, with each lenticular lens overlying a respective group of two or more adjacent columns of display sub-pixels.
Each lenticular lens can be associated with two columns of display sub-pixels to enable a user to observe a single stereoscopic image. Instead, each lenticular lens can be associated with a group of three or more adjacent display sub-pixels in the row direction. Corresponding columns of display sub-pixels in each group are arranged appropriately to provide a vertical slice from a respective two dimensional sub-image. As a user's head is moved from left to right a series of successive, different, stereoscopic views are observed creating, for example, a look-around impression.
FIG. 1 is a schematic perspective view of a known direct view autostereoscopic display device 1. The known device 1 comprises a liquid crystal display panel 3 of the active matrix type that acts as a spatial light modulator to produce the display.
The display panel 3 has an orthogonal array of rows and columns of display sub-pixels 5. For the sake of clarity, only a small number of display sub-pixels 5 are shown in the Figure. In practice, the display panel 3 might comprise about one thousand rows and several thousand columns of display sub-pixels 5. In a black and white display panel a sub-pixel in fact constitutes a full pixel. In a color display a sub-pixel is one color component of a full color pixel. The full color pixel, according to general terminology comprises all sub-pixels necessary for creating all colors of a smallest image part displayed. Thus, e.g. a full color pixel may have red (R) green (G) and blue (B) sub-pixels possibly augmented with a white and/or yellow sub-pixel and/or with one or more other elementary colored sub-pixels. The structure of the liquid crystal display panel 3 is entirely conventional. In particular, the panel 3 comprises a pair of spaced transparent glass substrates, between which an aligned twisted nematic or other liquid crystal material is provided. The substrates carry patterns of transparent indium tin oxide (ITO) electrodes on their facing surfaces. Polarizing layers are also provided on the outer surfaces of the substrates.
Each display sub-pixel 5 comprises opposing electrodes on the substrates, with the intervening liquid crystal material there between. The shape and layout of the display sub-pixels 5 are determined by the shape and layout of the electrodes. The display sub-pixels 5 are regularly spaced from one another by gaps.
Each display sub-pixel 5 is associated with a switching element, such as a thin film transistor (TFT) or thin film diode (TFD). The display pixels are operated to produce the display by providing addressing signals to the switching elements, and suitable addressing schemes will be known to those skilled in the art.
The display panel 3 is illuminated by a light source 7 comprising, in this case, a planar backlight extending over the area of the display pixel array. Light from the light source 7 is directed through the display panel 3, with the individual display sub-pixels 5 being driven to modulate the light and produce the display. The backlight 7 has side edges 7a and 7b, a top edge 7c and a bottom edge 7d. It has a front face from which light is output.
The display device 1 also comprises a lenticular sheet 9, arranged over the display side of the display panel 3, which performs a light directing function and thus a view forming function. The lenticular sheet 9 comprises a row of lenticular elements 11 extending parallel to one another, of which only one is shown with exaggerated dimensions for the sake of clarity.
The lenticular elements 11 are in the form of convex (semi-) cylindrical lenses each having an elongate axis 12 extending perpendicular to the cylindrical curvature of the element, and each element acts as a light output directing means to provide different images, or views, from the display panel 3 to the eyes of a user positioned in front of the display device 1.
A lenticular type imaging arrangement gives rise to multiple viewing cones. Within each cone, the set of different views is repeated. For multi-viewer displays, this is an advantage as it enables the full field of view to be filled with views. It may be especially advantageous for moving viewers to use a single viewing cone for example in a head tracking system which tracks the location of a single or small number of viewers.
The display device has a controller 13 which controls the backlight and the display panel.
The autostereoscopic display device 1 shown in FIG. 1 is capable of providing several different perspective views in different directions, i.e. it is able to direct the pixel output to different spatial positions within the field of view of the display device. In particular, each lenticular element 11 overlies a small group of display sub-pixels 5 in each row, where, in the current example, a row extends perpendicular to the elongate axis of the lenticular element 11. The lenticular element 11 projects the output of each display sub-pixel 5 of a group in a different direction, so as to form the several different views. As the user's head moves from left to right, his/her eyes will receive different ones of the several views, in turn.
A common issue for the lenticular-based autostereoscopic displays is that there is a reduced resolution in the 3D mode. Using multiple pixels under each lenticular lens element means that several views are generated simultaneously. This reduces the available resolution of autostereoscopic displays in 3D mode compared to the native resolution of the 2D display panel.
One known method to avoid this loss of panel resolution in the 3D mode is to generate the required different views in a time-sequential manner. This approach can be done for instance with a directional backlight component, which generates collimated light at different viewing angles at different moments of time.
If the switching speed of the backlight directionality is sufficiently fast and the generated light output directions cover the necessary range for multiple views, such a backlight can be employed to create a 3D display without requiring a lens, and with the full native resolution of the panel.
There are known examples of backlights which provide directional outputs. A two-view directional backlight concept is described in US 2009/7518663. The display device includes a display panel, a light redirection element for directing light through the display panel, and a light guide for directing light towards the light redirection element. Two light sources are coupled to the light guide to input light into the light guide in two directions. The light redirection element has a first groove structure and the light guide has a second groove structure so that light from the light sources are directed through the display panel with two angular distributions. The grooved light out-coupling structures are applied on the top of the lightguide.
Light is alternatively sent to the left and to the right eye of the observer synchronously with respective switching between images for the left and the right eye using a fast switching LCD.
Another example is a two-view directional backlight commercially available from the company 3M (trade mark). The design comprises a backlight component with prismatic groove out-coupling structures at the backplane of a lightguide, light sources situated at the two different sides of a lightguide, a light re-direction film and an LCD panel. The whole setup generates two views projected at different directions into the eyes of the observer. The views are generated time-sequentially, depending on the light source operating at one or another side of the lightguide.
An alternative approach is to use a backlight which generates spaced thin line light sources, with a display panel at a fixed distance from light sources, such that the light exiting each light source propagates at a different angular direction through different panel pixel elements.
Other known directional backlight designs use arrays of direct emitting fixed light sources and associated optical elements.
An array of light sources can for example be realized with a regular backlight and an LCD panel functioning as active barrier on top of the backlight, and a lenticular lens. A dynamic light source is then in the focal plane of the lens in order to generate a collimated output. This design usually results in low efficiency and reduced brightness. A similar concept may use fixed switchable OLED stripes and a lenticular lens on top.
FIG. 2 shows the use of a collimated backlight for controlling the direction from which a view can be seen. The backlight 7 comprises an array 30 of striped light emitters, a positive lens array 32 and a replica structure 34 between the lens array and the emitters. The lens array 32 collimates the light coming from the array 30 of thin light emitting stripes. Such a backlight can be formed from a series of emissive elements, such as lines of LEDs or OLED stripes. The extracted light from the emissive elements is in the form of an array of thin light emitting stripes spaced at around the pitch of the lens structure.
These solutions generally use active source steering (multiple addressable light sources or else a backlight combined with active barriers) in combination with fixed optical elements. They are therefore structurally complex.
Edge lit lightguides (otherwise known as waveguides) for backlighting and front-lighting of displays are inexpensive and robust. It would therefore be advantageous to base a collimated backlight component around the edge lit technology. However the known edge lit lightguides are designed to provide maximum uniformity of light output across the entire surface of the lightguide and are therefore not designed for generating an array of thin light stripes spaced at around the pitch of a combined lens.
FIG. 3 shows a schematic image of an edge lit lightguide 40. The lightguide comprises a waveguide material, such as a slab of solid material with a top face 40a, a bottom face 40b and lateral edges 40c. There are top and bottom edges which cannot be seen in FIG. 3, as the cross section of FIG. 3 is taken in the lateral side-to-side direction. The lightguide is generally rectangular in plan view. The top and bottom edges of the lightguide (at the top and bottom sides of the rectangle) are aligned to correspond to the top and bottom of the associated display, and the lateral edges (at the left and right sides of the rectangle) are aligned to correspond to the left and right sides of the associated display.
From the left side in FIG. 3, light is coupled in from a light source 42 and at the bottom of the lightguide several out-coupling structures 44 are placed. Light propagates under an angle θin inside the lightguide with height H. The out-coupling structures 44 in this example are drawn as half prisms with a half top angle σ, height h, and a width w.
The lightguide is formed as a dielectric slab made out of e.g. glass or polycarbonate. In the slab, total internal reflection at the borders keeps the light confined while the light propagates. The edges of the slab are typically used to couple in light and the small light out-coupling structures 44 locally couple light out of the lightguide.
US 2012/0314145 and US 2013/0308339 disclose a backlight for an autostereoscopic display using a special design of lightguide, which has a reflecting end face which functions as a lens. Different light sources provide light to the lightguide in different directions, and the lens functions create a collimated path within the lightguide in a particular direction. This is coupled out from the lightguide to create a particular backlight output direction. This requires a complicated lightguide structure.
The invention is based on the use of a more basic lightguide type backlight for use in an autostereoscopic display or privacy display, in particular to enable generation of a directional output, and in which time sequential operation is used to enable higher spatial resolution. A lightguide design is desired which can be kept thin and lightweight.